Hydrogen is an important industrial gas and is used in a number of applications such as ammonia synthesis, methanol synthesis, chemical hydrogenation, metal manufacture, glass processing and fuel cells. For many applications, the hydrogen must be free of impurities, so techniques for hydrogen purification have been developed. Ultra-high purity hydrogen is required for the manufacture of integrated circuits, creating the need for improved hydrogen purification processes. Commercially available hydrogen typically contains impurities including carbon monoxide, carbon dioxide, oxygen, nitrogen, water and methane, and these components must be separated from the hydrogen.
One method of purifying hydrogen uses palladium or palladium alloy membranes. The membranes are selectively permeable, and only hydrogen will pass through. The hydrogen is thus separated from the impurities. Typically the membranes function at temperatures in excess of 300° C., particularly 350° C.-400° C. A conventional hydrogen purification apparatus based on a hydrogen diffusion membrane comprises a chamber, a membrane located within the chamber, a heater for heating the membrane, and gas lines allowing the flow of gas streams into and out of the chamber.
When the membrane purification apparatus is turned off, either deliberately or by an external event such as a power outage, hydrogen must be removed from the system whilst the membrane is still hot (>300° C.) or hydrogen embrittlement of the membrane can occur leading to damaged, leaking membranes. Purge systems are used to remove hydrogen from the chamber containing the membrane. Typically nitrogen is introduced into the chamber in an attempt to flush out any hydrogen from the chamber.
Conventional purge systems can take at least thirty minutes to remove hydrogen from the chamber, by which time the membrane has cooled below 300° C. and there is a considerable risk of damage to the membrane. It is an object of the present invention to reduce the time needed to purge all hydrogen from the chamber of the hydrogen purification apparatus.
When the purification apparatus is re-started, it is desirable to remove nitrogen from the chamber as quickly as possible and switch to maximum hydrogen flow, so that the purification process can begin. It is a further object of the invention to reduce the time needed to start up a purged hydrogen purification apparatus.
In conventional purge systems, there is a risk that when the nitrogen flow is switched back to hydrogen flow, the resulting pressure surge can damage the membrane. It is a yet further object of the invention to reduce pressure surges within the apparatus so that the lifetime of the membrane is increased.